The invention relates to a multibeam antenna which is used for receiving micro waves from plural geostationary satellites.
Recently, many geostationary broadcasting satellites and geostationary communication satellites have been launched. The need for receiving micro waves from, for example, two adjacent satellites by using a single antenna and selectively using one of the received micro waves is increasing.
Conventionally, a multibeam antenna which receives micro waves from plural satellites is configured so that micro waves from plural satellites are reflected and focused by a single parabola reflector and the focused satellite signals respectively enter different primary radiators.
Horn type primary radiators (or feedhorns) are used as the primary radiators. When two satellite micro waves are to be received, for example, two horn type primary radiators are supported by an arm so as to be placed at the reflection and focusnce position of the parabola reflector. The elevation angles for the satellites with respect to the ground are different from each other. Furthermore, the degree of the difference in elevation angle is varied depending on the receiving areas. For each receiving area, therefore, the inclination of the horn arrangement of the primary radiators with respect to an axis which is in parallel with the ground must be adjusted.
Hereinafter, the inclination of the horn arrangement of primary radiators with respect to an axis which is in parallel with the ground is referred to as the inclination angle.
In the case where satellite signals to be received are linearly polarized, the inclination of each incident micro wave with respect to the ground is changed depending on the satellites and receiving areas. For each receiving area, therefore, the reception polarization angle of each primary radiator must be adjusted.
When the direction of the conventional multibeam antenna for linearly polarized waves is to be adjusted, therefore, the arrangement inclination angles of primary-radiator horns with respect to each satellite, and the reception polarization angles of primary radiators must be adjusted in accordance with the receiving area. This produces problems in that a mechanism for adjusting the angles is complicated in structure, and that the adjusting work is cumbersome.
Conventionally, a flaring horn type primary radiator is usually used as a primary radiator of an antenna for satellite broadcasting. Even when a parabola reflector has a small diameter of, for example, 45 cm .phi., the arrangement distance among the primary radiators can be sufficiently made large as far as adjacent satellites from which micro waves are to be received are separated from each other by an elongation of about 8 deg. Consequently, flaring horns of primary radiators can be adjacently arranged without interfering with each other. By contrast, in the case where adjacent satellites from which micro waves are to be received are separated from each other by a small elongation of 4 deg., the arrangement distance among the primary radiators is as small as about 25 mm. As a result, when such flaring horn type primary radiators are used, the radiator horns interfere or contact with each other and hence it is impossible to constitute a multibeam antenna, thereby producing a problem in that plural antennas respectively for satellites from which micro waves are to be received must be installed.
As discussed above, in a primary radiator of a 45-cm .phi. dual-beam antenna system which receives micro waves of the 12 GHz band from two satellites of an elongation of 4 deg., for example, the horn interval is about 25 mm. When a primary radiator of such an antenna is configured by a usual flare horn as shown in FIGS. 22(A) and 22(B), the aperture diameter is about 30 mm. Therefore, the antenna cannot be structurally configured. In order to realize such an antenna system, it is required to set the aperture diameter of a primary radiator to be 25 mm or less. In a circular waveguide designated as WCI-120 in EIAJ (Standard of Electronic Industries Association of Japan), the inner diameter of the waveguide is 17.475 mm. When such a waveguide is used, therefore, the horn has substantially a flare angle of about 0 deg. in consideration of the production process of an actual product. In other words, the horn has a circular waveguide section aperture as shown in FIGS. 23(A) and 23(B).
FIG. 22(A) is a front view of the conventional flare horn type primary radiator, and FIG. 22(B) is a section view taken along the line A-A' of FIG. 22(A). FIG. 23(A) is a front view of a conventional circular waveguide type primary radiator, and FIG. 23(B) is a section view taken along the line A-A' of FIG. 23(A).
In FIG. 22(A) and 22(B), 131 designates a flared waveguide which is disposed on a substrate 132. A feeding point 133 is configured by a printed circuit formed on the substrate 132, so as to be positioned at the center of the bottom face of the flared waveguide 131.
The circular waveguide type primary radiator shown in FIGS. 23(A) and 23(B) is a circular waveguide 135 in place of the flared waveguide 131. The other components are configured in the same manner as those of the flare horn type primary radiator of FIG. 22(A).
FIG. 24 shows the radiational pattern of the circular waveguide type primary radiator. In the case where the reflector is offset, the radiation angle of the primary radiator is about 40 deg. In the directional pattern of FIG. 24, the leakage power is large in the reflector irradiation, and the unevenness of the electric field in the reflector irradiation range is large. Therefore, the antenna gain is lowered.
Methods such as that in which the horn aperture diameter is reduced, that in which a helical antenna is used with supplying a power through a coaxial system, and that in which a traveling-wave type antenna such as a circular waveguide feed poly-rod antenna is used as a primary radiator may be used as means for solving the problems discussed above. In the conventional multibeam antenna, moreover, received-signal cables extending from converters for primary radiators are connected to an external switching device, and one satellite broadcasting program which is to be received is selected by controlling the switching operation of the switching device. This configuration involves problems in that the user must purchase such an external switching device, and that a wiring work and the like are required.
When an integral converter is configured by using plural primary radiators, substrate-printed probes 202 are formed on a single substrate 201 as shown in FIG. 29, and all other circuits also are disposed on the substrate 201. Each of the substrate-printed probes 202 comprises a horizontally-polarized-wave probe 202a and a vertically-polarized-wave probe 202b. The substrate-printed probes 202 are disposed in power feeding portions of plural (for example, two) primary radiator apertures 203, respectively. Signals output from the horizontally-polarized-wave probe 202a and the vertically-polarized-wave probe 202b are amplified by high-frequency amplifiers 203a and 203b, and then subjected to selection by horizontal/vertical changeover switches 204a and 204b. Signals which are selected by the horizontal/vertical changeover switches 204a and 204b are then subjected to further selection by a satellite changeover switch 205. The selected signal is amplified by a high-frequency amplifier 206, and then supplied to a frequency converter 207. The oscillation output of a local oscillator 208 is supplied to the frequency converter 207. The frequency converter 207 outputs, as an intermediate-frequency signal, a signal of a frequency which is equal to the difference in frequency between the signal from the high-frequency amplifier 206 and that from the local oscillator 208. The signal output from the frequency converter 207 is amplified by an intermediate-frequency amplifier 209. The amplified signal is supplied to the outside through a terminal 210.
The conventional multibeam antenna has problems in that the arrangement inclination angles of primary radiators must be respectively adjusted, and that the reception polarization angles of the primary radiators must be respectively adjusted.
The conventional multibeam antenna has a further problem in that, in the case where satellites from which micro waves are to be received are separated from each other by a small distance of, for example, 4 deg., flaring horn type primary radiators which are adjacently arranged contact or interfere with each other and therefore cannot constitute a multibeam antenna.
The conventional multibeam antenna has a further problem in that, in order to selectively receive a desired satellite broadcasting program, an external switching device, wirings for the device, and the like are required.
Furthermore, in the conventional primary radiator, a current supplied from a feeding point flows into a rear side through an edge portion of a horn aperture or that of a ground plane of a helical antenna, thereby causing the primary radiator to have radiational patterns in which radiation other than that to a reflector is large. As a result, the antenna gain is lowered.
When micro waves from plural satellites are to be received by the conventional converter for receiving micro waves from satellites, the substrate-printed probes 202 are set so that an axis which is in parallel with the ground in each area, the orbit inclinations of the objective satellites, and the polarization angles of the satellites coincide with each other. In this case, the converter is dedicated to the satellites from which micro waves are to be received. When converters corresponding to all satellites are to be produced, therefore, the converters cannot entirely share substrates, with the result that the productivity is impaired and hence the production cost of a converter is increased.